Universities in the States are a little different than others. You don't choose your major until you are in your 3rd year. Basically, you apply to get into a major your junior year and if accepted, you have a major. If you get turned down, you have to find a different major, or spend another semester or year trying to get in. That's why you see so many people not graduate after 4 years. The problem is that they do not know what to major in.

If you wonder how you can be a fourth year junior or a fifth year senior, you must understand that the number of credits you have completed represents what level you are. So a fourth year junior is someone who has been in school for four years, but has the same credits as a junior.